1. Field of the Invention
This invention relates to methods of and apparatus for coding television signals, and to apparatus for decoding television signals so coded.
2. Description of the Prior Art
A so-called common image format for high definition digital television is currently receiving attention. The concept envisages that for a variety of frame rates, such as 24 frames per second as used for film, and 25 and 29.97 frames per second as used for television in Europe, and the United States and Japan, respectively, the image size in terms of pixels in both the horizontal (number of samples) and vertical (number of lines) directions is constant. The concept is also applicable to a 30 frames per second television system. The number of pixels proposed is similar to that used for high definition digital television, and may be 1920 pixels horizontally and 1080 pixels vertically.
Suppose that common image format is to be used to make a film. In that case a common image format high definition digital television equipment is operated at 24 frames per second, and all operations, such as recording and editing, effected prior to transfer to film are carried out at 24 frames per second. The final edited version can then readily be transferred to film, as the frame rate is already correct.
Suppose now that common image format is to be used in Europe to make a television programme. In that case the common image format high definition digital television equipment is operated at 25 frames per second, and all operations, such as recording and editing, effected prior to broadcast transmission are carried out at 25 frames per second. The final edited version could of course then be transmitted as a high definition television signal, but until such a system comes into widespread use, conversion, for example to the 625-line system will be necessary prior to the transmission. However, it is to be noted that the necessary standards conversion can be quite simply effected because only spatial sub-sampling is required. No temporal interpolation is required, because the signal already has the required frame rate. The necessary standards converter is therefore relatively simple and cheap, and the quality of the final television signal is at least as good as that which would have been obtained if the whole process had been carried out using standard 625-line equipment.
Again, suppose that the same television programme is to be broadcast in Japan or the USA, where the frame rate is 29.97 per second. It should first be noted that a significant advantage of the common image format is that whatever the particular frame rate in use, the resulting signal can readily be recorded using a common helical-scan digital video tape recorder (VTR) in which the tape speed and the drum speed can selectively be controlled to correspond to the frame rate. In the present case, therefore, the television programme would have been recorded in Europe using such a VTR operating at a speed corresponding to 25 frames per second. The resulting tape could then be sent to Japan, for example, and reproduced using a similar VTR which, although normally operated at a speed corresponding to 29.97 frames per second would, for the purpose of reproducing this tape, be operated at a speed corresponding to 25 frames per second to reproduce the tape at the correct real time speed. The reproduced signal would then be standards converted to the 525-line 29.97 frames per second NTSC system used in Japan. This standards conversion involves changing the frame rate from 25 to 29.97 per second, and hence is preferably done using a relatively sophisticated motion-compensating standards converter. Such machines are expensive, but it is to be noted that with common image format, such a standards converter need only be used where there is conversion from high definition television to another television system having a different frame rate.
Common image format can also be applied to television systems in which the video signal is in analog form. In this case the band-width of the video signal is proportional to the frame rate, so that a bandwidth of 25 MHz may correspond to 25 frames per second, while a bandwidth of 30 MHz corresponds to 30 frames per second. When recording using a helical-scan analog VTR, a greater band-width and hence a higher frequency corresponds to a higher recording speed, and vice versa, but the spatial resolution per frame remains constant. In such a case it is envisaged that the audio signal will be in digital form, so references in this specification to an analog VTR mean a VTR which records video in analog form and audio in digital form.
Thus, it will be seen that common image format offers significant advantages in the production, processing, recording and standards conversion of video signals. In particular, common image format digital VTRs and analog VTRs can be provided which use a respective common track format and have the facility referred to above of operating with tape and drum speeds corresponding to various different frame rates. There is, however, a problem with a digitally recorded audio signal which, with the digital or analog video signal, forms the complete television signal. This problem is that for any given audio sample rate, such as 44.1 or 48 KHz, the number of audio samples associated with each video frame will vary depending on the frame rate.